bug#28439: suggestion: support case-independent xref-find-definitions prompt TAB

[Eli Zaretskii <eliz@gnu.org>]

> Cc: bug-gnu-emacs@gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 14 Sep 2017 02:04:10 +0300
> 
> >>     Since xref-find-definitions is willing to match names typed in all
> >>     lower case to mixed case names, TAB (show alternatives) should, too.
> >>     If there's not already a way to make TAB do so, then a way should be
> >>     added.
> > 
> > Dmitry, how about the patch below?
> 
> Sorry, what don't we want to follow the value of completion-ignore-case?

Because it has a much broader effect.  And because the issue here is
inconsistency between completion and actual matching in
xref-find-definitions.  They should be consistent, IMO.

> The fact that we have the option tags-case-fold-search (used in 
> find-tag-tag, among other places) probably means that some users prefer 
> not to ignore case.

Then maybe an alternative is to make tags-case-fold-search nil by
default?  Or make xref--read-identifier be case-insensitive if
case-fold-search is non-nil?

> Maybe we could add a similar xref-specific option on top, but I'm not 
> sure why completion-ignore-case is not good enough.
> 
> We could change its default to t, though.

Emacs-wide?  Or just when completing on identifiers?